w1891 - 3/11/2019 11:48
It depends on what you want to do with the lab numbers. Soil tests are only good because the value they return has been correlated to yield levels and corresponding fertilizer needs. Soil test levels are not an absolute “this is all of the P” in the soil values. So your 4” soil tests are only good for determining where on your previously researched scale you would fall. So on that 4” soil test you number is likely to be close to what is seen on a 6” test unless in a no-till situation where the soil is not mixed. University numbers had fudge factors and overestimated somewhat to make sure nutrients were not the limiting factor. Like mentioned before, trying to convert to lbs/acre is making things complicated and really tells you nothing. One of the controls of the research was the depth of the soil sample. So when you change that value the relationship between the value reported and the ability to predict yield limitations becomes even more blurry. I think an even more interesting question is "Is the research done in the 40's and 50's still valid when yields have doubled since then?" | 
Dummy question for ppm conversion
